This invention relates to photovoltaic (PV) power systems which utilize an inverter to convert the direct current (DC) output of PV arrays to alternating current (AC) and more particularly to improved methods and apparatus for measuring important system performance characteristics.
As it known in the art, PV refers to the conversion of incident solar radiation (insolation) to electricity. A PV cell (also known as a solar cell) can be provided from a large-area semiconductor pn junction diode with the junction positioned very close to the top surface. Since the output of an individual cell is relatively low, several PV cells are typically combined to form a PV module. In turn, multiple PV modules can be coupled to form PV panels and in turn again multiple PV modules can be coupled to form a PV array.
As it also known, a photovoltaic array exposed to solar radiation is capable of generating electrical power for a variety of purposes. One such purpose includes generating electrical power for a utility grid. PV arrays produce a DC voltage that is collected and combined and inversion is required for supplying AC loads or for utility-interactive operation. To thus convert the DC output voltage of the PV array to an AC voltage or current, a power inverter is used.
To interface the output of the PV array to a utility grid, a DC to AC inverter is needed to change the direct current voltage output of the PV array into, typically, a 60 Hz sinusoidal AC current waveform which feeds power to the utility grid.
The utility grid is a voltage source and as such power going into the grid can be changed by changing the magnitude of the current feeding the grid. Therefore, by changing the output current one can change the output power of the inverter within the constraints of the PV array feeding it.
For a condition of constant temperature and constant insolation there are an infinite number of voltage current pairs at which the PV array can operate. There is, however, one unique pair that produces maximum power from the PV array. This is called the Maximum Power Point (MPP). In an inverter connected to a power grid, it is desirable to operate at the MPP. A maximum power point tracker (MPPT) is used to ensure the system is operating at its MPP. The operating points are selected by changing the PV array load impedance. In the case where an inverter is connected to the PV array, the inverter is the load and its input impedance can be altered under microprocessor control. Modern inverters are quite good at locating the maximum power point for a given set of operating conditions. This is one of the characteristics that are utilized in the present invention.
Use of PV arrays connected to utility grid applications is becoming more common. It is not, however, a cost effective power generation source. There is, however, a trend of rapidly dropping prices for increased production volume for PV arrays. Due to rising concern over the greenhouse effect, it is desirable to reduce electricity generation through combustion. Photovoltaic arrays are a clean energy source that can displace combustion and reduce greenhouse emissions. For this reason, it is desirable to accelerate the introduction of PV arrays. This can be done through increased demand. One way to increase demand is by increasing the value of a PV system beyond just the raw power that is produced.
One growing market for PV systems are in school mounted systems. Typically these systems incorporate a rudimentary Data Acquisition System (DAS) that sends data into a school classroom. These DASs can increase the cost of a system by as much as 20%. Usually the price is paid because the data can be used in the classrooms to teach science. Usually the students are very motivated when using this type of laboratory tool. Thus, the capability added by a DAS adds considerable value to the PV system which enhances the usefulness of the PV system as a teaching tool.
Conventional data acquisition systems, however, have several drawbacks. As mentioned above they are relatively expensive. Also data acquisition systems typically do not provide the capability to monitor harmonic distortion and perform PV array I-V curve traces.
It would, therefore, be desirable to provide an inverter system which provides substantially the same functionality provided by conventional DAS supplemented inverter systems at a reduced cost and with the added functionality of monitoring harmonic distortion and acquiring I-V curve traces.
The present invention reduces the cost of PV system data acquisition capability by a factor of from 5 to 10 while dramatically increasing functionality. In particular it adds the capability to monitor harmonic distortion and perform PV array I-V curve traces. Further it can eliminate the need for a relatively costly insolation transducer. The present invention reduces cost by utilizing circuitry that already exists in the inverter. It had not heretofore been recognized that there exists a multitude of redundancy between components in the inverter and in the external DAS and that by modifying an inverter, capability beyond that of an industry standard DAS could be included for substantially lower cost. Thus, as will be described herein below, an inverter modified in accordance with the present invention provides capability beyond that of an industry standard DAS at a relatively low cost.
In accordance with the present invention, a method for measuring parameters of a photovoltaic (PV) power system includes the steps of operating an inverter for converting direct current (DC) to alternating current (AC) including the steps of measuring input DC voltage to the inverter; measuring input DC current to the inverter; measuring output AC voltage from the inverter; measuring output AC current; calculating, from the input DC voltage, the input DC current, the output AC voltage and the output AC current; maximum power point of the inverter; calculating, from the input DC voltage and the input DC current, solar insolation; and calculating, from the maximum power point and the solar insolation, temperature of a solar cell. With such an arrangement, an inverter is provided with an integrated low cost data acquisition system (DAS).
In accordance with another aspect of the present invention, a method for measuring parameters of a photovoltaic power system includes the step of operating an inverter for converting direct current (DC) to alternating current (AC) including the steps of disconnecting an output of the inverter from any device connected thereto; disconnecting an input of the direct current fed to the inverter; discharging any residual charge on a large fully discharged capacitor; connecting the input of the direct current across a large fully discharged capacitor; measuring input DC voltage to the inverter; measuring input DC current to the inverter; and providing current voltage data to provide a current-voltage curve tracer. With such an arrangement, an inverter is provided, when off line with a power grid, having a capability to gather information of current voltage curve tracing which can then be used to provide other information.
In accordance with another aspect of the present invention, an inverter includes a first switch to disconnect an output of the inverter from any device connected thereto; a second switch to disconnect and connect an input of the inverter from direct current fed to the inverter; a discharge device to discharge any residual charge on a large capacitor; a voltage sensor to measure input DC voltage to the inverter; a current sensor to measure input DC current to the inverter; and a data recorder to provide a current voltage trace to provide a current-voltage curve tracer. With such an arrangement, a current-voltage curve tracer is provided to provide current-voltage curve data that may then be used to calibrate a maximum power point tracker or alternatively sense array shading.